Spectroscopy Characteristics and Color-Influencing Factors of Green Iron-Bearing Elbaite

Abstract

The color-influencing factors and spectroscopy of 22 green elbaite samples were investigated using X-Rite SP62 spectrophotometry, ultraviolet–visible (UV–Vis) spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and infrared spectroscopy. The chromogenic components iron and manganese were found in the green elbaites; however, the bivariate correlation analysis indicated that the Mn content had no impact on the color, whereas the Fe content significantly affected both the lightness and the hue of green elbaites. The primary factors influencing the color of tourmaline were the absorption band at 720 nm caused by the Fe2+ d-d transitions and the 300 to 400 nm wide absorption edge extending to the visible range due to the O2−-Fe3+ charge transfer. Infrared spectroscopy indicated that the color of tourmalines was also influenced by their structure. As the degree of Y and Z octahedral distortion in the tourmaline lattice increased, the lightness of the tourmaline decreased and its color deepened. The hydroxyl region of the infrared spectra of green elbaite showed three distinct peak positions representing two types of hydroxyl vibrations: O1H at the W position and O3H at the V position. The O1H vibrations are attributed to YLiYAlYAl and XNa or X position vacancy, while the O3H vibrations are assigned to ZAlZAlYAl and ZAlZAlYLi. The presence of three peaks in the hydroxyl vibrational region of the IR spectra indicated that these samples were iron-bearing elbaites.